The present invention generally relates to an Auxiliary Power Unit (APU) of the type capable of providing power to an aircraft as needed. In particular, the present invention is directed to lightweight and high-energy start system for providing power to the APU to initiate APU start up.
Conventional APU start-up systems (electric, hydraulic and pressurized air start systems (PASS) are not utilized to provide in-flight emergency power because of the significant size and weight requirements. In one system developed for the F-22, a jet fuel/air-stored energy system was utilized for both APU starting and in-flight emergency power. This system when used for APU starting and emergency power has increased complexity, weight and cost relative to a system designed for an APU start-only system. It is considered desirable to minimize the weight, complexity and cost if at all possible.
In a known APU start-up system, the turbine power module (TPM) combustor utilized for driving an APU gearbox was designed to operate in both fuel-rich and lean-burn modes. The fuel-rich mode reduced air consumption during extended emergency power operation, while the lean-burning mode eliminated the normal maintenance required by carbon accumulations during APU ground starts.
A rich-burn mode requires the use of high-temperature combustor liner to maintain fuel-rich combustion at low power levels. Such a liner would not be needed for a high-powered, lean-burning APU starting system. By removing the combustor liner, liner insulation and transition liner, it would not only reduce cost and weight but, more importantly, it may significantly reduce the TPM""s sensitivity to temperature transients. The relatively large thermal mass of the combustor housing may be better able to handle any high-temperature combustion transients.
Because the xe2x80x9ctouchxe2x80x9d temperature should not exceed 500xc2x0 F. for safety considerations after single ground starts, the need for an outside insulation blanket would be eliminated. A further disadvantage of conventional start-up systems resides the complexity of the combustor head. Fuel-rich combustor processing requires the use of a duplex fuel nozzle to provide a wide turndown ratio. However, since the APU starter system only operates at one power level and does not require large droplets to maintain clean fuel-rich combustion at high power levels, the duplex nozzle configuration and associated divider valve utilized in an APU start system providing emergency power can be eliminated, resulting in significant cost and weight savings.
There is a clear need for an improved APU starter system capable of meeting the streamlined requirements of ground starting while operating in single, lean-burning mode. Such a starter-system should be of minimum size and weight and be able to function with a limited amount of compressed air and fuel.
In one aspect of the present invention, an APU starter system is disclosed. The system includes a source of pressurized air and a source of jet fuel. The system further includes a turbine power module attached to an APU. An air flow passageway joins the source of pressurized air to the turbine power module. A fuel flow passageway joins the source of jet fuel to the turbine power module, and a separate valve assembly located in each flow passageway controls the flow of compressed air and jet fuel into the turbine power module.
In another aspect of the invention, an APU starter system includes a source of pressurized air, comprising at least one storage vessel. The system further includes a source of jet fuel, comprising a fuel tank. A turbine power module is attached to an APU and an air flow passageway joins the at least one storage vessel to the turbine power module. A fuel flow passageway joins the fuel tank to the turbine power module. A separate valve assembly located in each flow passageway controls the flow of compressed air and jet fuel into the turbine power module.
In a yet further aspect of the invention, an APU starter system formed in accordance with the present invention includes a source of pressurized air, comprising at least one storage vessel and a source of jet fuel comprising a fuel tank. The system further includes a turbine power module attached to an APU with an air flow passageway joining the at least one storage vessel to the turbine power module and a fuel flow passageway joining the fuel tank to the turbine power module. A modulating valve assembly located in the air flow passageway and a control valve located in the fuel flow passageway control the flow of compressed air and jet fuel into the turbine power module.
In another aspect of the present invention, a method of starting an APU includes the step of energizing a control valve located in an air flow system between a source of pressurized air and a turbine power module. The method further includes the step of energizing a control valve located in a fuel flow system between a source of jet fuel and the turbine power module. The method also includes the step of igniting the mixture of air and fuel within the turbine power module to create a steam of hot gases; and directing the steam of hot gases onto turbine blades for rotating the blades to drive the APU through a gearbox.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.